lthread_poll assertion error

I'm getting this assertion error: lthread/src/lthread_sched.c:252: lthread_run: Assertion `lt_write != ((void *)0) || lt_read != ((void *)0)' failed.

Trying this with some basic code:

```
pollfd p;
memset(&p, 0, sizeof(p));
p.fd = fd;
p.events = POLLIN;
lthread_poll(&p, 1, 1000000);
```

If I substitute these lines of code for:

```
lthread_wait_read(fd, 0);
```

Then it waits just fine.
